Chapter 1 What’s New for FortiOS 5.0 : Other new features : New interface features - DHCP server and authentication : Reserving, assigning and blocking MAC addresses
  
Reserving, assigning and blocking MAC addresses
While adding a DHCP server to an interface you can select MAC Address Control list and select Create New to configure the DHCP server to:
Always assign the same IP address to a device according to its MAC address (Reserved IP address)
Block access to a device according to its MAC address (MAC address filtering)
The default action is to Assign an IP address to a device.
Figure 92: Reserving an IP address for a device with a specific MAC address